Shinji Fuyushima is an A-rank Trapper and captain of the Fuyushima Squad, holding the A-rank #2 position within Border. His squad includes combatant Isami Tōma and operator Risa Maki. He possesses long, slicked-back brown hair styled in a short ponytail under his head, with facial hair consisting of sideburns and a goatee. His personality is easygoing and calm, and he occasionally refers to himself as "this old man," a trait potentially linked to being older than most Border agents.

Fuyushima joined Border during its debut period, initially enlisting as an Engineer. He was later recruited by Maki to form Fuyushima Squad with Tōma. During the Large-Scale Invasion Arc, he collaborated with Motokichi Kinuta to set traps in designated Danger Zones for defense. He subsequently assisted A-rank agents in battles against the Neighbors Hyrein and Mira.

In the B-Rank Wars Arc, he attended a strategic meeting regarding forecasted invasions. When Azuma proposed deploying S-rank agent Tsukihiko Amō, Fuyushima expressed confusion until Azuma clarified the intent to utilize Amō’s Side Effect. During the invasion, Fuyushima provided support to Kazama Squad and other agents operating outside. After the battle, he assessed the situation as a victory but maintained teleportation readiness for city defense as a precaution.

His name "Fuyushima" (冬島) translates to "winter island," while "Shinji" combines the characters for "act with care/be cautious" (慎) and "next/secondary/following" (次). Outside Border duties, he enjoys games, sukiyaki, and Lego blocks.